Due to relatively high instances of system inertia and delay in automotive transmissions, exclusively using feedback control of various components in automotive transmissions may not be sufficient for certain transient schemes. In such transient cases, feedforward control may be used to anticipate system changes. For example, mixed feedforward and feedback control can be used for a smooth upshift in an automatic transmission without causing significant “feel” issues for the driver, thereby improving overall shift quality.
An upshift operation in an automatic transmission typically involves a first clutch (or releasing element) that disengages from a first gear while a second clutch (or applying element) engages a second gear. A third clutch remains engaged during the upshift operation. Therefore, at least two clutches are typically engaged during each gear level of an automatic transmission. For example, an upshift in a 4-speed automatic transmission may be a 1-2 shift, a 2-3 shift, and/or a 3-4 shift.
Referring now to FIG. 1, clutches in automatic transmissions are typically actuated by pressure from fluid such as oil. Oil fills the clutch cavity including a piston and displaces the piston to engage the clutch. Automatic transmission systems also typically include accumulators 10 for each clutch. As oil fills the cavity including the clutch it also fills the accumulator 10. The accumulator 10 includes a spring 12 that is compressed when a piston 14 in the accumulator 10 is displaced by pressure from the oil. The oil begins to displace the piston 14 after all of the empty space in the accumulator 10 is filled with oil and the pressure of the oil increases. Therefore, accumulators 10 reduce the sensitivity of torque variations during the torque phase of an upshift.
A solenoid controls the flow rate of oil to the accumulator 10 and the associated clutch. The duty cycle of a control signal applied to the solenoid determines the fraction of time during which the solenoid valve is open or closed. Thus, the solenoid controls the flow rate and ultimately the pressure of the oil. Also, the accumulator 10 makes the pressure response slower and more difficult to predict. The calculated volume of oil present in the accumulator 10 at a given time is an indicator of the capacity of the clutch at that time. Therefore, the calculated volume can be used for control purposes.
However, the calculated volume may vary throughout the life of the transmission due to wear of the clutch and/or other factors. Transmission control systems have been developed that adjust the duty cycle of the solenoid over the operating life of the transmission to thereby account for changes in the system and to maintain smooth shifting. For instance, applicant's patent application, entitled “TARGET VOLUME BASED TORQUE PHASE CONTROL DURING UPSHIFT”, Ser. No. 11/222,066, filed Sep. 8, 2005, discloses such a system. Applicant's patent, entitled “MODEL BASED KICKDOWN SHIFT METHOD FOR CLUTCH TO CLUTCH SHIFT TRANSMISSIONS WITH ACCUMULATORS”, U.S. Pat. No. 6,978,201, which issued Dec. 20, 2005, also discloses such a system.
The fill volume (i.e., the volume of oil at which an applying element begins to produce torque during a shift) of each clutch is used by the control logic of these systems to effectuate gear shifts. However, the fill volumes can vary over the lifetime of the vehicle due to wear and/or other factors. As a result, gear shifting may be detrimentally affected due to inaccurate fill volume values stored in memory in the system. Accordingly, there remains a need for a means of accurately measuring updating stored values of the fill volume of a transmission.